This invention relates to a semiconductor light emitting device having a light emitting layer provided by semiconductor layers that are largely different in lattice constant therebetween, e.g. where gallium-nitride based compound semiconductor layers are formed overlaid on a sapphire substrate. More particularly, this invention relates to a semiconductor light emitting device which is structured to have semiconductor layers having a light emitting layer and so on, that are less affected by lattice mismatch in the crystalline structure and hence almost free of crystalline disorders.
The conventional semiconductor light emitting device for emitting light, e.g. bluish light, is structured as shown in FIG. 3. That is, the semiconductor light emitting device includes a sapphire substrate 21. The sapphire substrate 21 has thereon a low-temperature buffer layer 22, e.g. of GaN, an n-type layer (cladding layer) 23 of n-type GaN grown by epitaxy, an active layer (light emitting layer) 24, e.g. of InGaN-based compound semiconductor having a bandgap energy smaller than that of the cladding layer so as to define an emission-light wavelength, and a p-type layer (cladding layer) of p-type GaN. Note that in this specification "InGaN-based" means a chemical composition containing In and Ga in a variable ratio therebetween. The light emitting device has a p-side electrode formed on a surface thereof and an n-side electrode on a surface of the n-type layer 23 exposed by partly etching the overlying semiconductor layers. Incidentally, the n-type layer and the p-type layer, in many cases, have respective AlGaN-based compound semiconductor layers on an active layer 23 side, in order to enhance carrier confining effects, wherein "AlGaN-based" means a chemical composition containing Al and Ga in a variable ratio therebetween.
As stated above, the conventional semiconductor light emitting device formed of a gallium-nitride based semiconductor e.g. for bluish light emission is provided with a GaN layer or an AlGaN-based compound semiconductor layer as n-type and p-type layers. However, the lattice constant is different between sapphire, i.e. 4.6 angstrom, and GaN, i.e. 3.19 angstrom, or Al.sub.0.3 Ga.sub.0.7 N, i.e. 3.13 angstrom. Moreover, the lattice constant of GaN or Al.sub.0.3 Ga.sub.0.7 N is different from that of the InGaN-based semiconductor for the light emitting layer. Therefore, there is a tendency of incurring lattice mismatch in the n-type or p-type layer structure, impeding the flow of electric current. Further, there is a problem that cracks are caused between the crystals to reach the light emitting layer due to the difference in crystal lattice constant, with the result that the device has a lower light emitting efficiency. In particular, the n-type layer, in most cases, is formed to a layer thickness of approximately 4 to 5 .mu.m, which accelerates accumulation of crystalline disorders and causes further crystal mismatch.
In this conventional structure, the emission-light wavelength is determined by the bandgap energy of the material of the light emitting layer 24. Where In.sub.x Ga.sub.1-x N is employed for the light emitting layer 24, the increase in mixture crystal ratio x of In (lowering in bandgap energy for the light emitting layer) increases the wavelength of light emitted. Conversely, when the ratio x is decreased (the bandgap energy is increased), the wavelength of light emitted is shortened. In order to obtain a wavelength covering from 450 nm for blue light (Zn may be doped at the In mixed crystal ratio x of around 0.2, though blue light is available with the ratio x of approximately 0.4) to a green light portion (the ratio x is approximately 0.5), an InGaN-based compound semiconductor is utilized. When providing a shorter wavelength light than the above, a material such as GaN or an AlGaN-based compound semiconductor is employed for the light emitting layer. In this manner, the double hetero-junction structure has the light emitting layer sandwiched between the semiconductor layers that are different in lattice constant from that of the light emitting layer. In particular, the InGaN-based compound semiconductor layer tends to become unstable in crystalline texture, as the thickness thereof increases. When the InGaN-based compound semiconductor layer is sandwiched between the semiconductor layers that are different in lattice constant therefrom, the different lattice mismatch tends to cause cracks in the crystal and impede electric current flow. Thus, there is a problem that the light emitting efficiency is lowered.
On the other hand, there is a disclosure by Japanese Provisional Patent Publication (Kokai) No. H5-110138 and Japanese Provisional Patent Publication (Kokai) No. H5-110139 that describes a method, in order to form a Ga.sub.p Al.sub.1-p N layer, of forming thin GaN and AlN layers alternately. In this method, thin GaN and AlN layers are alternately overlaid so that the content ratio of Ga and Al of these layer becomes coincident with that of the Ga.sub.p Al.sub.1-p N layer. This technique, however, is a method to exclusively provide a Ga.sub.p Al.sub.1-p N layer and not applicable to other film materials than GaN and AlN due to difficulty in growing a crystal with high property, as is described on page 3, column, 3, lines 29 to 31 in Japanese Provisional Patent Publication No. H5-110138.